This invention relates to cellular wireless communication systems and more particularly to a system and method for increasing utilization of an allocated frequency in a fixed cellular wireless system.
1. Background
Traditionally, cellular wireless communication systems have focused on mobile telephony and wireless local area network (LAN) communications. In such systems a base station at a fixed location has an omni-directional antenna for bi-directional communication with a mobile terminal as it travels throughout the cell area covered by the base station. The terminal also has an omni-directional antenna to enable reception of communication regardless of location within the cell. Omni-directional receivers tend to be relatively polarization insensitive.
Microwave radio frequencies in the GHz range are being allotted to wireless service providers through allocated frequency licenses. A license will conventionally cover a frequency range and the licensee will attempt to optimize utilization of the range through frequency multiplexing (so-called frequency plans) whereby the frequency range is subdivided into narrow bands or channels. Notwithstanding this subdivision it is to a licensee""s advantage to optimize utilization of each frequency within the allocated frequency range.
In broadband communication of the type contemplated by the present invention, for example asynchronous transfer mode (ATM) networks or Internet Protocol (IP) networks, both the base station which serves as an ATM switch and usually the customer or user at a remote site are in a fixed location. This factor allows the utilization of directional transceivers at both the base station and the remote site.
2. Prior Art
U.S. Pat. No. 5,668,610 which issued Sep. 16, 1997 to Bernard Bossard et al. and assigned to Cellularvision Technology and Telecommunications, L.P. describes a transmitter array utilizing polarization diversity sub-cells. In this patent a directional antenna is utilized for transmitting television signals to a substantially circular cell area. The central base station utilizes a directional transmitter such that the cell area is divided into two substantially equal sectors. Transmission in one sector is vertically polarized whereas transmission in the second half of the cell is horizontally polarized. The receivers in each sector are configured to accept the type of polarization directed to it and to reject the opposite type of polarization. This, if effect, substantially doubles the utilization of each frequency.
The present invention takes advantage of the polarization diversity described in U.S. Pat. No. 5,668,610 by utilizing transceivers at the base station which divides the cell area into more than two sectors. This results in greater use of a frequency allocation by multiplying the utilization of each frequency by the number of sectors employed up to a practical limit of between 2 and 8 (typically 4). This is referred to frequency reuse. In the present invention the polarization in each sector is orthogonal to the polarization in each adjacent sector.
Additionally, the present invention contemplates bi-directional communication between the remote site and the base station. The polarization for transmission in each direction is also orthogonal such that, for example, radiation between the base station and the remote site (downstream or downlink) is vertically polarized while radiation from the remote site to the base station (upstream or uplink) is horizontally polarized. Radiation in adjacent sectors is reversed.
In this way, the utilization of the frequency license is increased by a factor of N or 4 in the aforementioned four-sectored cell.
Also contemplated by the present invention is orthogonal polarization wherein the radiation in the receive and transmit directions are, for example, at linear +45 degrees and linear 45 degrees, respectively. This orthogonal relationship leads to improved bi-directional system performance against rain fade.
Therefore, in accordance with a first aspect of the present invention there is provided a cellular wireless communications system comprising a base station having a transceiver for communications with a remote site over a wireless link, the transceiver having transmission means to generate N sectored radiation patterns where N is greater than 2 and the sum of the sectored patterns covers cell area; and transceiver means at the remote site, the remote site being at a location within one of the sectors and the transceiver means having a directional antenna aimed at the base station for receiving transmissions therefrom. Radiation within the one of the sectors is orthogonally polarized with respect to radiation in adjacent sectors.
In one embodiment of the invention the cell area is substantially circular and N is an even number, preferably four. Additionally, the remote site is in a fixed location and communication between the base station and remote site is bi-directional.
According to a further preferred embodiments of the invention the cellular wireless communication system is for broadband communication utilizing an asynchronous transfer mode (ATM) network or an IP network.
In accordance with a second aspect of the present invention there is provided a method of increasing the utilization of a defined frequency in a cellular wireless communication system for bi-directional communication between a base station and a remote site over a wireless link. The method comprises: providing a transceiver at the base station having means to generate N sectored radiation patterns where N is greater than 2 and the sectored patterns combine to cover a cell area; providing a transceiver at the remote site located in one of the sectors, the transceiver having a directional antenna aimed at the base station for receiving radiation-transmitted therefrom; and employing orthogonally polarized radiation between adjacent sectors.